AS the least known of the key 20th-century dictators, Francisco Franco was due a political biography of this size and scope.

Paul Preston's ``Franco: A Biography'' promises to unravel decades of official misinformation by following Franco ``step by step and day by day.'' By the 1,000th page, some readers may feel they have trod every one of them. But this long march is worth the price.

To Franco's propagandists - of whom the dictator himself was first among equals - he stands with El Cid as one of Spain's great warrior heroes. To detractors, he was a small man, living in the shadow of Hitler and Mussolini, remarkable only for presiding over half a million deaths and 39 years of political repression.

The strength of this book is the quality of evidence it brings to bear on disputed points. Along with documents, diaries, and speeches, Preston draws on the testimony of new witnesses, both among Franco supporters and political foes. In the process, he provides new answers to some old puzzles.

One such issue: Could Franco have won the Spanish Civil War without the backing of Hitler and Mussolini?

The bombers that Hitler sent to Franco helped ``turn a coup dtat going wrong into a bloody and prolonged civil war.'' But Preston notes that Franco was a gifted general in his own right. As a young officer in the Spanish Legion in Morocco in 1916, he led assaults himself and was genuinely popular with his men. Later at age 33, he became the youngest general in Europe. In the early months of the Spanish Civil War, Franco's ``unquenchable optimism'' kept up morale among his own men and consolidated his authority with fellow rebels elsewhere in Spain, Preston writes.

On the other hand, Franco's management of the war was clearly more with an eye to ensuring his own postwar authority than to bringing a swift end to the conflict. Battle by battle, Preston notes points where an advantage could have been pressed, but wasn't. Franco needed a long, slow war, a war of attrition, to destroy as many political foes as possible. He wrote to an impatient Italian ambassador: ``I will take the capital not an hour before it is necessary: First I must have the certainty of being able to found a regime.''

Another puzzle: Given Franco's debts to Germany and Italy's fascist dictatorships, how did Franco keep Spain out of World War II?

Observers as keen as the late Dean Acheson credited Franco's fear of Hitler as part of the equation. This Preston denies. There was no evidence of German threats against Spain, he writes. Spain was more useful to Germany neutral. ``Above all, Franco's neutrality rested on the appalling economic and military plight of a Spain shattered by the Civil War, a disaster from which the Caudillo thus derived enormous benefit.''

As Spain starved and Europe was at war, Franco was writing a semiautobiographical novel glorifying his own battle to save Spain from the ``foreign poisons of liberalism, freemasonry, socialism, and Communism.''

At war's end, Franco's propagandists worked overtime rewriting his regime's war record. But Franco early recognized that what would break Spain's international isolation and the implacable hostility of the United States State Department was the cold war. Preston documents how Franco mobilized a Spanish lobby in Washington, arguing Spain's value to the West in the battle against communism. ``Franco knew that the change in the State Department's attitude was running behind that of the American military and financial establishments.'' A $25 million loan from the Chase Manhattan and National City Banks of New York, followed by the 1953 US Bases Agreement, confirmed his conviction.

Perhaps the most interesting puzzle is that Franco oversaw policies that modernized Spain's economy and eroded his own police state. Two years after his death in 1975, ``every vestige of dictatorship had disappeared.''

Preston suggests that Franco just slipped away. He spent more and more time shooting partridges (5,000 in one outing) and fishing for tuna. His decision to turn key ministries over to technocrats was a ``realistic political judgment.'' In the end, Franco was convinced by his ministers' technical arguments because he no longer understood them. Meanwhile, Spain had become an obvious target for foreign capital. ``[T]he Franco regime's contribution to economic growth was the decades of hardship which left Spanish workers willing to work long hours for low wages.''

There have been many books written on the Spanish Civil War with more poetry and passion, but few with more attention to detail and regard for truth.